How Nerve Signals Actually Travel

Touch a hot pan and you pull your hand back before you have even consciously registered the pain. That speed is not an accident. Nerve signals travel as tiny electrical pulses, built for exactly this kind of urgency.

A single neuron is a specialised cell built to do one thing extremely well. It carries a signal rapidly along its length and passes it on to the next cell in the chain. The electrical pulse At rest, a neuron holds a small electrical difference across its outer membrane. When the neuron is triggered, that difference briefly flips in a wave that sweeps along the neuron's length, called an electrical impulse. This impulse travels extremely fast. It is essentially the same basic signal whether it is carrying a sensation of pain, an instruction to a muscle, or anything else the nervous system handles. Why some signals travel faster than others Many neurons are wrapped in a fatty layer called myelin, arranged in short segments along their length with tiny gaps in between. Myelin acts as insulation. The electrical impulse effectively jumps from gap to gap rather than travelling continuously along the whole membrane. This jumping makes myelinated neurons dramatically faster than unmyelinated ones. This is why reflex pathways and urgent sensory signals, like the pain of touching something hot, rely heavily on myelinated neurons. The synapse: handing the signal on A neuron does not physically touch the next neuron in the chain. Between them sits a tiny gap called a synapse. When the electrical impulse reaches the end of a neuron, it triggers the release of a chemical messenger, called a neurotransmitter, into this gap. The neurotransmitter crosses the gap and binds to the next neuron. If enough of it arrives, it triggers a fresh electrical impulse in that neuron, continuing the signal onward. Why the synapse matters so much The synapse is not just a connector. It is a control point. Because the next neuron only fires if enough neurotransmitter arrives, synapses can strengthen, weaken, or even block a signal entirely, depending on other signals arriving at the same time. This is part of how the nervous system processes and filters information, rather than simply relaying every signal unchanged from one end to the other. Putting the chain together A sensation such as touching something hot travels as an electrical impulse along a sensory neuron. It crosses one or more synapses, and continues as fresh impulses along further neurons, until it reaches the spinal cord or brain. A response, such as pulling your hand away, travels the same way in the opposite direction. The speed of this entire chain, electrical impulse, synapse, electrical impulse, is what makes a reflex feel instantaneous. What a doctor checks A doctor testing nerve function is, in effect, checking whether this chain of electrical impulses and synapses is working normally along a particular pathway. Reflex testing. Tapping a tendon triggers a reflex that depends on fast, myelinated sensory and motor neurons and their synapses in the spinal cord. A reflex that is absent, reduced, or exaggerated points toward where along that specific chain something may be affected. Sensory testing. Light touch, pinprick, and vibration are each carried by somewhat different types of neurons, so testing several types can help localise a problem to a specific kind of nerve fibre. Nerve conduction studies. Where more detail is needed, a specialised test can directly measure how fast an electrical impulse travels along a specific nerve. This can reveal damage to the myelin or to the neuron itself. When this system is affected Conditions that damage myelin can slow or block nerve signals considerably, even when the neurons themselves are intact, producing symptoms such as numbness, weakness, or altered sensation. This is explored further in the lesson "Weakness or Numbness".

A nerve signal is a brief electrical pulse that travels along a neuron and then crosses to the next neuron using a chemical messenger at a tiny gap called a synapse.